[Boatanchors] Constant-voltage audio transformers

[Dave Maples]

John: If you are already set, that's fine.  For those that aren't, however,
I do recommend the Quam TBL-70 and TBL-25 as suitable candidates along these
lines.  They typically offer 0.5, 1.0, 2.0, 4.0, and 8.0 watt taps.

In any case, as you cruise junk boxes next summer at your favorite hamfest,
this might be something you want to keep an eye out for.

Thanks,
Dave WB4FUR

-----Original Message-----
From: J. Forster [mailto:jfor at quikus.com]
Sent: Sunday, November 25, 2012 2:53 PM
To: Dave Maples
Cc: boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Constant-voltage audio transformers


Yes, I understand the theory.

The only 70.7 V one available from RS is a unit with a minimum power tap
of 5 Watts, or 1000 Ohms. That is identical with the little audio output
one I already have.

So, if I have to shop for a transformer, I might as wello look for a
little tube output transformer, which is a better match.

Actually, I have been offered somethng pretty close to what I need, so am
all set.

Best,

-John

==================


> All: After reading the request for a transformer to couple a BC-221 meter
> to
> low-Z headphones, I thought it might be helpful to some folks to read some
> information about 70.7-volt audio transformers.  These devices are
> actually
> very handy for solving some of these kinds of problems.  They are also
> cheap, which is always attractive.
>
> Why are they called "70.7-volt transformers"?  It's because they are
> designed such that if the primary is fed with 70.7 VAC from an audio
> amplifier, the transformer and associated speaker will draw a specific
> amount of power from that audio amplifier.  The amount of power that will
> be
> drawn is basically only a function of the wattage tap selected on the
> transformer.
>
> This arrangement makes it very, very easy for a single audio amplifier to
> drive a large number of speakers at different power levels while ensuring
> that the amplifier is properly loaded (which, as all of you know, is
> especially important with vacuum-tube amplifiers).  All the installer has
> to
> do is to add up the wattages drawn by each speaker in parallel, and make
> sure that the total wattage matches the amplifier power rating.
>
> How does that help us?  Well, the primary of the transformer may actually
> be
> a very high-impedance load, depending on the wattage tap chosen.  That's
> what we are looking for.  Unlike the audio technician, though, we are
> going
> to have to calculate what we want.  That's really easy, though.
>
> 1. If you recall, P=(E*E)/R, where P= power (watts), E=RMS volts, and
> R=resistance in ohms.
>
> 2. By multiplying both sides by (R/P), we find that R=(E*E)/P.
>
> 3. Since E = 70.7 VAC, E*E=70.7*70.7=5000
>
> 4. R, therefore, will equal 5000 divided by the wattage tap you choose.
> Choosing the 1-watt tap (assuming you have the proper impedance on the
> secondary) will give you a 5000-ohm load.  Choosing the 2-watt tap will
> give
> you 2500 ohms.  Choosing the 8-watt tap will give you 625 ohms, which is
> just about perfect for an R-390 or similar.
>
>
> You can also find 25-volt speaker transformers, just as you can find 70.7
> volt speaker transformers.  The same principles apply, except that in (3)
> above the value of E*E is 625, not 5000.  This means that the 1-watt tap
> on
> a 25-volt transformer will present a 625-ohm load to a source, which again
> is a pretty close match for an R-390 or similar.
>
>>>>The point is that you can apply either to your situation, and you can
>>>> do
> so KNOWING what load you are presenting to your source.  You don't have to
> guess.<<<
>
>
> The Quam TBL-25 and TBL-70 are great little transformers for this purpose,
> and are quite cheap.
>
>
>
> One other thing to remember: These transformers are NOT designed for
> constant DC current to flow through them.  They are AC-only devices.
>
>
> I hope this is helpful.
>
> Regards,
> Dave WB4FUR
>
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